Portable locks for securing bicycles frequently include a crossbar and a U-shaped shackle that is removably coupled to the crossbar via a locking mechanism seated in the crossbar. These locks, often referred to as “U-locks” owing to the shape of the shackle, are typically able to provide favorable levels of security at a relatively low weight in comparison to certain other forms of portable locks, such as those involving chains or articulating shackles. However, these U-locks are not without their drawbacks. For example, the large size and rigid nature of the U-shaped shackle may render the lock difficult to carry when not in use, and may hinder the use of the lock in tight or cramped spaces.
Another area of concern for U-locks is susceptibility to saw attacks, as the elongated and exposed shackle provides the attacker with a relatively large space in which to work the saw. When the saw is used to cut the shackle at the bend, the elongated legs naturally provide a large lever arm with which the locking mechanism can be pried. The shackles may also facilitate twisting attacks, in which a pry bar placed between the crossbar and the shackle is twisted to expand the opening formed by the cut.
As is evident from the foregoing, existing U-locks suffer from a variety of drawbacks and limitations associated with the U-shaped shackles thereof. For these reasons among others, a need remains for further improvements in this technological field.